In the electrolytic refining of metals, especially the electrolytic refining of copper, the deposition of copper at the cathode from the electrolyte, especially from an impure copper anode is carried out with a current density usually between 150 and 300 amperes per m.sup.2. The individual electrolysis baths are connected in series, i.e. one after the other.
For a given current flow, the production rate per unit time of cathodic copper, i.e. the amount of copper deposited at the cathodes per unit time, is a function of the number of cells and the current efficiency.
It has been recognized that it is possible to obtain an increase in the production rate by raising the number of electrolysis cells. The disadvantage of this technique is that it involves increased investment costs for additional electrolysis tanks, rails, piping, electrolyte, pumps and baths. Furthermore, it requires an increase in the copper stock and the use of rectifiers and transformers of greater output.
Another way of increasing production, already recognized in the art, is to increase the current. High current densities have, however, the disadvantage that the overvoltage at the cathode increases disproportionately so that undesirable metals, for example lead, antimony, bismuth, selenium, iron and arsenic, are deposited at the cathode in addition to the desired metal, namely, copper. Then it is necessary to avoid the deposition of such impurity metals, the current density is, as has been recognized in the art, limited to about 300 amperes per m.sup.2.
A slight increase in the production rate can be obtained by increasing the current efficiency.
As long as one operates with current densities below 900 amperes per m.sup.2, the increase in current is the simplest and least expensive method of raising production rate as long as the deposition of impurity metals at the cathode is acceptable. If such deposition is not acceptable, the use of increased currents must be accompanied by attempts to lower the overvoltage at the cathode.
It is known in the art (see French Pat. No. 1,412,438, English Pat. No. 1,157,686 and U.S. Pat. No. 3,864,227) to provide a current reversal process which has the function of eliminating passivation characteristics at the anode.